In producing electrical energy from pressurized water nuclear reactor systems, the reactor provides heat for producing steam that is used to drive a turbine. The fuel elements or rods that are present in the pressurized water nuclear reactor comprise a container or cladding material that contains a nuclear fuel such as uranium dioxide, the fuel being sealed in the container.
While various types of cladding materials for use in forming the container have been proposed, the most prevelant are zirconium based alloys, because of the strength, corrosion resistance and low neutron absorption provided by these alloys under operating conditions.
Designers of such fuel elements have traditionally purposely added small quantities of alloying elements and/or interstitial elements such as oxygen or carbon to zirconium to increase the strength of zirconium-base cladding materials. Typically, specifications in use specify an oxygen content of a zirconium alloy cladding material, such as a Zircaloy, of between about 1000 to 1550 parts per million by weight. In order to achieve such amounts, oxygen must purposely be added to a Zircaloy ingot used in forming cladding.
While the addition of oxygen to the specified range increases the strength of Zircaloy, a desirable property, such oxygen presence also results in a reduction in the resistance of the cladding to pellet-clad interaction failures (PCI). As described in my copending application Ser. No. 919,943, filed Oct. 17, 1986, released fissile materials from the nuclear fuel during operation of a nuclear reactor cause a problem of stress corrosion and possible failure of the metallic tubular cladding. The chemical reaction of the zirconium alloy tubing with the volatile fisslle materials, coupled with cladding operating stresses can produce stress corrosion cracking of the zirconium alloy cladding and resultant penetration of the tube wall. To prevent pellet-clad interaction failures, it has been proposed to use a liner tubing on the inside of the cladding, such as a zirconium liner between the fuel pellets and a Zircaloy cladding. Such tubular linings are of the type described in U.S. Pat. No. 4,200,492 and U.S. Pat. No. 4,372,817. U.S. Pat. No. 4,200,492 discloses a nuclear fuel element comprising a zirconium alloy tube with a barrier of sponge zirconium metallurgically bonded to the inside surface of the alloy tube. The sponge zirconium barrier has a thickness of 1 percent ot 30 percent of the thickness of the alloy tube to protect the alloy tube from effects of a nuclear fuel contained therein. The liner of sponge zirconium is said to remain soft during irradiation and to minimize localized strain on the nuclear fuel element, and to protect the alloy tube from stress corrosion cracking or liquid metal embrittlement. The sponge zirconium liner contains 1000 to 5000 ppm of impurities, with the oxygen content of the impurities being about 200 to 1200 ppm. U.S. Pat. No. 4,372,817 is a companion case to U.S. Pat. No. 4,200,492 and discloses a nuclear fuel element similar to that of the earlier patent. The claims require a zirconium alloy tube having constituents other than zirconium in an amount greater than 1000 ppm, while the continuous barrier layer is zirconium metal of impurity content less than 500 ppm, of which the oxygen content is less than about 200 ppm. Such liners are described as barrier layers which are used to protect a zirconium alloy cladding from problems associated with pellet-clad interaction affects.
The use of a high purity low oxygen zirconium liner on the inside of a Zircaloy cladding provides appreciable ductility so that when small cracks are formed on the inner surface of the Zircaloy cladding, as a result of interaction of the cladding with the fuel and fission products, the liner blunts the propagation of the crack to reduce the problems associated with pellet-clad interaction. Such liner tubing is however expensive and results in added cost to a user.
Other approaches to solving the problem of pellet-clad interaction have also been proposed. For example, in copending application Ser. No. 589,390 filed Mar. 14, 1984, now U.S. Pat. No. 4,675,153 in the name of Raymond F. Boyle and assigned to the assignee of the present invention, a composite nuclear fuel rod cladding tube resistant to pellet-clad interaction is described which includes two concentric layers of zirconium base alloys metallurgically bonded to each other. The outer tube comprises a conventional zirconium base alloy having high strength and excellent aqueous corrosion resistance, such as Zircaloy-2 or Zircaloy-4, while the inner layer is a zirconium alloy having about 0.2 to 0.6 weight percent tin, 0.3 to 0.11 weight percent iron, and up to about 350 parts per million oxygen. A fuel element using this composite cladding contains a pressurized inert gas, such as helium, pressurized to about 2 to 5 atmospheres (29.4-73.5 pounds per square inch, psi). Such helium pressurization is primarily for the purpose of providing better heat transfer in the fuel rod.
As another example, in copending application Ser. No. 790,005, filed Oct. 22, 1985 in the names of John P. Foster and George P. Sabol and assigned to the assignee of the present invention, which application is incorporated by reference herein, a cladding tube is described which is composed of a single zirconium base alloy provided preferably with a cold worked and stress relieved microstructure throughout. This cladding has both excellent aqueous corrosion resistance and excellent pellet-clad interaction crack propagation resistance as well as good structural mechanical properties, due to the elemental composition and the presence of no more than 10 volume percent of recrystallized equiaxed grains in the microstructure, with preferably no observable recrystallized grains being present. The impurity content of the zirconium base alloy is less than 1500 parts per million by weight with the oxygen impurity content being less than 600 parts per million, and preferably less than 400 parts per million.
It is an object of the present invention to provide a nuclear fuel element that uses a zirconium or zirconium alloy unitary cladding material, without the need for a liner of coating thereon, or special grain structures, required for the cladding.